Cluster tools used in the semiconductor processing of wafers or other workpieces typically comprise a central wafer handling chamber, or transfer chamber, surrounded by a number of process chambers within which various processes are carried out on the wafers (e.g., deposition, etching, doping, annealing and oxidizing). A robot is provided in the transfer chamber for moving the wafers within the cluster tool. The transfer chamber is typically isolated from each of the process chambers by gate valves. The gate valves can be opened to allow the robot to transfer the wafers between the transfer chamber and the process chambers.
For reduced pressure processing, transfer of wafers between the transfer chamber and the process chambers can be carried out at reduced pressure, thereby eliminating the need to pump down the process chamber after each wafer transfer and increasing wafer throughput. It is important to prevent cross-contamination between the chambers during wafer transfer. Gaseous (homogeneous) cross-contamination takes place when gases from one chamber are convectively or diffusively transported to another chamber, thereby contaminating the other chamber. For example, an oxidizing species used to grow a silicon dioxide or oxynitride film in one process chamber could contaminate another process chamber in which an oxygen-free environment is desired, such as for epitaxial deposition. Additionally, particulate (heterogeneous) particulate can also arise during wafer transfer.
In the past, attempts have been made to control the flow of gases between the chambers, and thereby reduce gaseous cross-contamination, by creating a pressure differential between the transfer chamber and the process chamber to be accessed prior to opening the gate valve therebetween. The intent is that, when the gate valve is opened, the gases will flow from the higher pressure chamber to the lower pressure chamber to prevent contamination of the higher pressure chamber. Various pump systems have been devised for creating pressure differentials between the chambers.
One problem with such systems is that creating a pressure differential between the chambers does not necessarily ensure that the desired gas flow will occur upon opening of the gate valve. This is due in part to the dependence of the pressure differential-induced flow on the absolute pressures in the chambers. In addition, a back flow or back diffusion of the gases into the (initially) higher pressure chamber can occur upon opening of the gate valve.